聚合

http://www.postgresql.org/docs/9.3/static/functions-aggregate.html

 array_agg：

聚合函数，返回一个array，相当于oracle的wm_concat：

digoal=# select array_agg(fooid) from foo;
    array_agg    
-----------------
 {4,5,7,6,1,8}
(1 row)

setof转换为array：

digoal=# select array(select fooid from foo);
      array      
-----------------
 {4,8}
(1 row)

bit_and，bit_or：

bit_and：所有非空的输入值的按位与。

bit_or：所有非空的输入值的按位或。

postgres=# select bit_and(1|3);
 bit_and 
---------
       3
(1 row)

postgres=# select bit_and(1&3);
 bit_and 
---------
       1
(1 row)

postgres=# select * from ct;
 id | rowid | attribute | value 
----+-------+-----------+-------
  1 | test1 | att1      | val1
  2 | test1 | att2      | val2
  3 | test1 | att3      | val3
  4 | test1 | att4      | val4
  5 | test2 | att1      | val5
  6 | test2 | att2      | val6
  7 | test2 | att3      | val7
  8 | test2 | att4      | val8
(8 rows)
postgres=# select bit_or(id) from ct where id<3;
 bit_or 
--------
      3
(1 row)

string_agg：

多行数据字符串拼接：

postgres=# select string_agg(attribute,',') from ct;  
               string_agg                
-----------------------------------------
 att1,att2,att3,att4,att1,att4
(1 row)

json_agg，xmlagg：

xmlagg需要在编译安装postgresql的时候“HINT:  You need to rebuild Postgresql using --with-libxml.”。

postgres=# select json_agg(value) from ct;  
                             json_agg                             
------------------------------------------------------------------
 ["val1","val2","val3","val4","val5","val6","val7","val8"]
(1 row)

postgres=# select pg_typeof(json_agg(value)) from ct;     
 pg_typeof 
-----------
 json
(1 row)